[KingMob]

Persistent data structures are very cool. While generally slower, they're within a factor of 2x-4x of mutable structures in general, and for certain domains, are actually faster.

E.g., if your mutable code has to make many array copies, you've negated any performance advantage waiting for memory. Another great one is checking for differences in tree structures; you can replace deep equality checking with higher-up pointer comparisons, which is why Clojurescript fits so well with virtual DOM techniques like React.